For years scientists have theorized that there is a massive amount of water hiding in the Earth’s mantle–the layer below the crust and above the outer molten core. This new study led by Steve Jacobsen and his colleagues offers the first direct evidence that these theories are correct.

“Using a network of 2,000 seismometers placed across the entire US, they were able to “listen” to the speed of the waves made by earthquakes as they moved through the varying depths of the Earth’s crust,” explains Gizmodo.

This technique allowed them to “hear” the difference between water and rock. They discovered a rock called ringwoodite that’s capable of soaking up water like a sponge when subjected to tremendous pressure, and it’s probably a key factor in the creation of magma.

“Geological processes on the Earth’s surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight,” said Jacobsen. “I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades.”